The investigators will test the hypothesis that markers of vascular endothelial dysfunction will be exaggerated acutely with an extreme high sodium diet compared to an extreme low-sodium diet. The investigators will compare patients with postural orthostatic tachycardia (POTS) to healthy control subjects.

The primary analysis will involve a non-parametric, paired, Signed Rank test of RH-PAT between all subjects (POTS & control subjects) on the high sodium diet vs low sodium diet

Secondary Outcome Measures:

fasting glucose and insulin levels [ Time Frame: after steady state has been reached (>5 days) on low and high sodium diets ] [ Designated as safety issue: No ]

Secondary analyses will include non-parametric paired comparisons between low sodium and high sodium diets of continuous outcome measures (outlined in section 7.3). Secondary comparisons might also assess differences in the dietary sodium induced changes between POTS patients and control subjects.

A blood pressure cuff will be placed on one upper arm (study arm; non-dominant), while the contralateral arm will serve as a control (control arm).

RH-PAT probes will be placed on one finger (finger II, III, or IV) of each hand (same finger on both hands). The fingers on either side of the one with the probe will be separated using soft sponge rings.

Continuous recording of pulsatile blood volume responses from both hands will be initiated.

After a 10-min equilibration period, the blood pressure cuff on the study arm will be inflated to 60 mm Hg above systolic pressure for 5 min. The cuff will then be deflated to induce reactive hyperemia, PAT recording will be stopped.

Other Name: EndoPat

Device: Calf Blood Flow in Reactive Hyperemia (CBF-RH)

Calf blood flow (CBF) will be determined using venous occlusion plethysmography and calibrated mercury strain-gauges during reactive hyperemia after a 5 min of ischemia of the distal limb. Strain-gauges will be applied to the widest part of the non-dominant calf (~10 cm below patella). Participants will remain quietly supine for 10 min with legs elevated on foam pads above the right atrium to achieve stable baseline measurements of CBF. The venous occlusion cuff is inflated for 4 seconds at 8 seconds intervals, while monitoring the change in resistance in the system, pressure inside the measuring cuff, and 5-10 determinations are performed

Other Name: Hokansan strain guage venous plethysmography

Device: Evaluation of Forearm-Mediated Dilation

The arm will be kept extended and immobilized at heart level. Brachial artery diameter will be measured using a high resolution ultrasonography using a linear array probe with a 5 to 17 MHz frequency range. The brachial artery will be imaged in longitudinal sections, 5-10 cm proximal to placement of an occlusion cuff in the dominant forearm just below the antecubital fossa. The probe will be held with a stereotaxic holder with micrometer movement capabilities.

Other Name: Ultrasound

Low Sodium Diet (POTS & Controls)

Participants will consume a very low sodium diet (10 mEq/day) for 4-5 days prior to study day.

A blood pressure cuff will be placed on one upper arm (study arm; non-dominant), while the contralateral arm will serve as a control (control arm).

RH-PAT probes will be placed on one finger (finger II, III, or IV) of each hand (same finger on both hands). The fingers on either side of the one with the probe will be separated using soft sponge rings.

Continuous recording of pulsatile blood volume responses from both hands will be initiated.

After a 10-min equilibration period, the blood pressure cuff on the study arm will be inflated to 60 mm Hg above systolic pressure for 5 min. The cuff will then be deflated to induce reactive hyperemia, PAT recording will be stopped.

Other Name: EndoPat

Device: Calf Blood Flow in Reactive Hyperemia (CBF-RH)

Calf blood flow (CBF) will be determined using venous occlusion plethysmography and calibrated mercury strain-gauges during reactive hyperemia after a 5 min of ischemia of the distal limb. Strain-gauges will be applied to the widest part of the non-dominant calf (~10 cm below patella). Participants will remain quietly supine for 10 min with legs elevated on foam pads above the right atrium to achieve stable baseline measurements of CBF. The venous occlusion cuff is inflated for 4 seconds at 8 seconds intervals, while monitoring the change in resistance in the system, pressure inside the measuring cuff, and 5-10 determinations are performed

Other Name: Hokansan strain guage venous plethysmography

Device: Evaluation of Forearm-Mediated Dilation

The arm will be kept extended and immobilized at heart level. Brachial artery diameter will be measured using a high resolution ultrasonography using a linear array probe with a 5 to 17 MHz frequency range. The brachial artery will be imaged in longitudinal sections, 5-10 cm proximal to placement of an occlusion cuff in the dominant forearm just below the antecubital fossa. The probe will be held with a stereotaxic holder with micrometer movement capabilities.

Other Name: Ultrasound

Detailed Description:

The study will involve a crossover design in which each subject will be assessed (as below) while on a very low-sodium (10 mEq/day) diet compared with a very high-sodium diet. These acute dietary interventions will be part of the parent study ("Dietary Salt in Postural Tachcyardia Syndrome" funded by R01 HL102387) for 4-5 days at the time of the study. Dietary success will be assessed using a 24h urine for sodium and creatinine as a part of the parent study.

Blood will be drawn and collected in a fasting state for future assay and analysis of the following tests:

Increase in heart rate ≥30 beats/min with position change from supine to standing (10 minutes)

Chronic symptoms consistent with POTS that are worse when upright and get better with recumbence Control Subjects

Healthy, non-obese, non-smokers without orthostatic tachycardia

Selected to match profiles of POTS patients (gender, age)

Not using vasoactive medication

Age between 18-60 years

Male and female subjects are eligible.

Able and willing to provide informed consent

Exclusion Criteria:

Overt cause for postural tachycardia (such as acute dehydration)

Inability to give, or withdrawal of, informed consent

Pregnant

Other factors which in the investigator's opinion would prevent the subject from completing the protocol.

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Please refer to this study by its ClinicalTrials.gov identifier: NCT01550315